The present invention relates to an eddy current defect-detecting system, and more particularly to an eddy current defect-detecting system for metal tubes which is capable of determining defects on inner and outer surfaces and detecting the depth of a defect based on the phase of a flaw or defect of a defect signal which phase varies with the conditions of the defect formed on a metal tube, that is, the location and depth of the defect.
Prior eddy current defect-detecting systems have a test coil with a high-frequency current flowing therethrough. For defect detection, the test coil is inserted into a metal tube for electrically measuring a variation in the amplitude of an eddy current generated by a flaw (hereinafter referred to as a "defect") present on the metal tube. Since the variation in the eddy current (impedance variation) is quite small, it is amplified by a detector (hereinafter referred to as a "bridge") and an amplifier and then converted into a voltage. The signal includes a defect-indicative signal component and noise due to wobbling of the coil caused when the latter is inserted into the metal tube being tested. This test method is known as an amplitude method which indicates the shape and depth of a defect with one signal for evaluation of the defect.
However, the conventional eddy current defect-detecting systems fail to tell a defect with a large thickness reduction in a narrow area from a defect accompanying a small thickness reduction in a wide area, and hence are liable to judge a metal tube suffering only from a small thickness reduction and still usable as requiring replacement.